The present invention relates to a time-division multiplex pulse code modulation (PCM) system for use in combining multiple digital data streams into a single high-speed output bit stream, and more particularly to a multiplexer apparatus having a line code converting function built into it.
Generally in a digital transmission system, the consecutive occurrence of the same code, "0", "1", in a line code stream would make it difficult for each repeater to extract timing, or invite the wandering of the D.C. level of the pulse string under the effects of D.C. component restriction, in turn resulting in such adverse effects as intersymbol interference. To avoid these undesirable consequences, such line codes are selected as would be able to keep the consecutive occurence of the same code within a certain number of bits in any input code stream and further to suppress the mark density fluctuation.
For conversion into such line codes, there are currently in use multi-value coding (including bipolar coding, for instance) and block coding (including nBmB coding or nB1C coding) among other conversion techniques.
In an optical digital transmission system where the light intensity is modulated with digital signals, multi-value coding is undesirable in view of its low S/N ratio, and block coding is usually employed instead. Among various block coding techniques, nBmB coding, by which each block of n bits are converted into one block code consisting of m bits, is considered to give the most desirable line codes.
In a usual nBmB coding process, a single high-speed bit stream once multiplexed is again separated by a serial-to-parallel converting circuit into n bit streams, each set of whose n mutually corresponding bits constitute an address word for a programmable read only memory (P-ROM), wherein a table of codes is registered with each of multiple n-bit words. The P-ROM output code words, each of which is composed of m bits corresponding to each one of the address words. This m-bit code word or block is further converted into a single signal stream by a parallel-to-serial converting circuit. For an example of this process, reference may be made to "A 140 Mbit/s Optical Transmission System with 8 km Repeater Spacing and Line Section Length of 96 km" by K. Mouthaan and J. R. Schlechte, Conference Proceedings of Optical Communication Conference, Amsterdam, Sept. 17-19, 1979.
Such an nBmB coding process, however, requires an n-bit serial-to-parallel converter and an m-bit parallel-to-serial converter, both operating at high speed, besides an nBmB line coder, and consequently invites greater hardware size, higher cost and lower reliability.